The state-of-charge (SOC) of a secondary battery can be correlated to the open-circuit voltage VO, which is the battery terminal voltage when the current path is cut off, also known as the electromotive force or the open voltage. Consequently, it is theoretically possible to estimate the state-of-charge by estimating the open-circuit voltage VO. However, it takes time for the terminal voltage of the secondary battery to level off after cut-off of the power supply (completion of charge/discharge). In order to estimate open-circuit voltage VO correctly, a prescribed time is needed from the completion of charge/discharge because right after charge/discharge and during charge/discharge it is impossible to estimate open-circuit voltage VO correctly. As a result, known devices have estimated open-circuit voltage VO.
In the state-of-charge estimating device described in Japanese Kokai Patent Application No. 2004-178848, for example, current I and terminal voltage V of the secondary battery and an adaptive digital filter computation from a battery model formula are used to estimate the battery parameters in the battery model formula to estimate open-circuit voltage VO. Based on the estimated open-circuit voltage VO and a previously derived relationship between an open-circuit voltage and a state-of-charge (SOC), state-of-charge (SOC) is estimated from the estimated open-circuit voltage VO.